(1) Field of the invention.
This invention relates to a speech analysis system comprising means for converting an input analog speech signal into a digital speech signal, means for storing segments of said digital speech signal, means for transforming each segment into a sequence of spectrum components, which means comprise means for performing a discrete Fourier transformation, whereby a series of amplitude spectrums each consisting of a sequence of spectrum components is produced.
(2) Description of the prior art.
Such a speech analysis system is generally known in the art of vocoders. As an example reference may be made to IEEE Transactions on Acoustics, Speech and Signal Processing, Vol. ASSP, No. 7, Aug. 1978, pp 358-365. In the prior art system disclosed therein the amplitude spectrums are supplied to a harmonic pitch detector for detecting the pitch period from the frequency distances between the peaks of the envelope of each amplitude spectrum.
It has been mentioned, that basically, a pitch detector is a device which makes a voiced-unvoiced (V/U) decision, and, during periods of voiced speech, provides a measurement of the pitch period. However, some pitch detection algorithms just determine the pitch during voiced segments of speech and rely on some other technique for the voiced-unvoiced decision. Cf. IEEE Transactions on Acoustics, Speech and Signal Processing, Vol. ASSP-24, No. 5, Oct. 1976, pp 399-418.
Several voiced-unvoiced detection algorithm are described in said last publication, based on the autocorrelation function, a zero-crossing count, a pattern recognition technique using a training set, or based on the degree of agreement among several pitch detectors. These detection algorithms use as input the time domain or frequency domain data of the speech signal in practically the whole speech band, while for pitch detection on the contrary the data of a low pass filtered speech signal are generally used.